It is known that, at the rear of a turbofan engine, the cold stream (bypass stream) and the hot stream (core stream) flow in the same direction downstream of said turbofan engine and come into contact, not only with one another, but also with the ambient air. Since the velocities of said streams differ from one another and from the ambient air velocity, penetrating fluid shearing actions result between said streams and between the latter and the ambient air, said fluid shearing actions generating noise, referred to as “jet noise” in the aeronautical field.
To attenuate such jet noise, it has already been considered to generate turbulence at the boundaries between the fluids having different velocities. It has thus already been proposed to make notches in the outlet edge for the hot stream and/or in the outlet edge for the cold stream (see, for example, GB-2 289 921). Such notches are distributed around the periphery of said outlet edge and each of them generally has the at least approximate shape of a triangle whose base is coincident with the corresponding outlet edge and whose vertex is situated in front of this outlet edge. These notches are generally referred to as “chevrons” in the aeronautical field. Of course, said “chevrons” are in pairs separated by a “protuberance”.
These known notches and protuberances are effective in attenuating the jet noise; however, they have the drawback of generating considerable drag.
Moreover, document GB-2 372 779 states that jet noise attenuation is not necessary in cruising flight and that, consequently, it is advantageous, in particular as far as drag is concerned, to make said protuberances movable so that they can adopt:                either a projecting deployed position, used on take-off and landing, in which they are able to attenuate the jet noise;        or a retracted position, used in cruise, in which they exert no jet noise attenuation action.        
To achieve this, GB-2 372 779 discloses that said outer annular surface or said inner annular surface of said rear section of the cowl surrounding the hot stream generator is made partially movable through the action of an actuating mechanism specially provided for this purpose. It will be noted that, when said protuberances are in the deployed position, the notches are through-going (not closed off by the annular surface which does not bear the notches) and that, when said protuberances are in the retracted position, said notches are blind (closed off by that one of said annular surfaces which does not bear the notches).
Thus, the technical lesson given by document GB-2 372 779 contains the following two propositions:    a) jet noise attenuation is obtained only with through-going notches; and    b) no jet noise attenuation is possible if the notches are blind.